The present invention relates to a coating apparatus for coating a substrate such as a semiconductor wafer or an LCD (liquid crystal display) substrate with a chemical solution such as a resist solution.
A photolithography technology is employed in a manufacturing process of a semiconductor device. In the photolithography technology, a surface of a semiconductor wafer is coated with resist, followed by exposing the coated resist layer to light in a predetermined pattern and subsequently developing the light-exposed pattern. As a result, a resist film of the predetermined pattern is formed on the wafer. Then, a conductive film is formed over the entire surface of the wafer, followed by selectively etching the conductive film so as to form a predetermined circuit pattern. A coating-developing system as disclosed in, for example, U.S. Pat. No. 5,664,254 is used in such a series of resist processing.
In recent years, the line width of a semiconductor device circuit is required to be on the order of sub-microns. In this connection, severer demands are being directed to further improvements in the uniformity and thickness of the resist film. To meet these requirements, a spin coating method is mainly employed nowadays for coating a surface of a wafer with a resist solution. In the spin coating method, the thickness of the resist film is controlled by adjusting the rotating speed of the wafer. Where, for example, it is desired to decrease the thickness of the resist film, the wafer is rotated at a high speed during the coating step.
In the conventional resist coating apparatus, however, it is mechanically impossible to increase as desired the rotating speed of a spin chuck supporting the wafer. Naturally, the rotating speed of the wafer is limited by the rotating speed of the spin chuck, with the result that it is impossible to make the thickness of the resist film smaller than a certain level. Particularly, in coating the surface of a large wafer having a diameter of, for example, 8 inches or 12 inches, the spreading speed of the resist solution tends to be lowered on the wafer surface, making it more difficult to form a thin resist film on the wafer surface.
Further, immediately after change in the process conditions such as a desired thickness of the resist film and the kind of the resist solution, the resist film formed on a first substrate after the change tends to be nonuniform in thickness, resulting in failure to meet a target value. In such a case, it is necessary to remove the coated resist film from the wafer and to coat again the wafer with a resist solution.
To overcome the above-noted difficulty, used is a resist solution having a suitable viscosity. The viscosity of the resist solution is determined by, for example, a mixing ratio of a solvent such as a thinner, which is added to the resist solution. A resist solution having the viscosity adjusted appropriately is put in a tank, and the tank is incorporated in the resist coating-developing system. In this fashion, the thickness of the resist film is made variable over a wide range.
However, the resist solution is poor in its compatibility with a thinner, with the result that, if a mixture of the resist solution and the thinner is left to stand within the tank over a long period of time, these resist solution and thinner are separated to form an upper layer and a lower layer, respectively, within the tank. This makes it necessary to use a relatively large apparatus for stirring the resist solution-thinner mixture within the tank, which is contradictory to the miniaturization of the apparatus.
It should also be noted that it is impossible to coat uniformly a surface of a large wafer with a resist solution by simply controlling the viscosity of the resist solution. Where, for example, a mixing ratio of the solvent is excessively increased in an attempt to increase the flowability of the resist solution on the wafer surface, the thickness of the resultant resist film is rendered unduly thin. In conclusion, with increase in the diameter of the wafer, it is very difficult to determine appropriately the viscosity of the resist solution for obtaining a desired thickness of the resist film.
As described above, the conventional resist coating apparatus is not satisfactory in that it is difficult to rotate the wafer at a desired high speed in the resist coating step, making it difficult to form a uniform resist film of a desired thickness particularly when a resist film is formed on a wafer having a large diameter. It is also difficult to form a uniform resist film of a desired thickness by means of controlling the viscosity of the resist solution.
In order to decrease the thickness of the resist film formed on the wafer surface, used is a resist solution advantageous in flowability on the wafer surface. As described previously, the viscosity of the resist solution is determined by, for example, the mixing ratio of a solvent such as a thinner. Also, a tank housing a resist solution having the viscosity adjusted appropriately by controlling the mixing ratio of the solvent is incorporated in the coating system, making it possible to control as desired the thickness of the resist film over a wide range.
However, as already pointed out, a resist solution is not compatible with a thinner. If a resist solution-thinner mixture is allowed to stand within the tank, the resist solution and the thinner are separated to form upper and lower layers. This makes it necessary to use a relatively large apparatus for stirring the solution-thinner mixture housed in the tank, which is clearly contradictory to miniaturization of the apparatus.
It should also be noted that it is necessary to replace the tank itself housing a resist solution every time the target thickness of the resist film is changed. It is also necessary to prepare in advance a plurality of tanks housing resist solutions differing from each other in viscosity in order to comply with the change in the target thickness of the resist film. Naturally, it is highly troublesome to take care of such a group of tanks. What should also be noted is that, even if a resist solution of the same viscosity is used, the thickness of the resist film formed is not necessarily the same because the thickness in question is affected by, for example, the daily differences in the environmental conditions. In such a case, it is desirable to use a resist solution having a slightly different viscosity. However, a tank housing a resist solution of a desired viscosity is not necessarily included in the group of tanks incorporated in the system.
To reiterate, in the conventional resist coating apparatus, a tank housing a resist solution having a desired viscosity must be newly used every time the target thickness of the resist film to be formed is changed, making it necessary to prepare in advance a plurality of tanks housing resist solutions differing from each other in viscosity in order to comply with a change in the target thickness of the resist film. Also, it is difficult to cope with a slight change, which is caused by the daily change in the environmental conditions, in the thickness of the resist film formed.